The introduction of catalytic mufflers precipitated considerable concern that the catalyst in these anti-pollution devices could be poisoned by metals from fuels and lubricant additives. With regard to crankcase oils for combustion engines, attention has been focused on eliminating or at least reducing the metal content in the additive package used therein in an effort to obviate the crankcase oils as a source of catalytic muffler poisoning. It is the general consensus that if the metal content in lubricating oils attributable to additives can be maintained below about 1 wt. % (sulfated ash basis) such low ash lubricating oils are compatible with today's pollution technology. In addition, low ash oils provide performance advantages in reducing spark plug fouling contributing less to higher octane requirements of gasoline than high ash formulations (substantially above 1 wt. % sulfated ash). The principal candidates as a combination dispersant-VI improver in low ash formulations are the polyalkylmethacrylates of a molecular weight between about 25,000 and 2,500,000. However, it was found that at the low sulfated ash levels (0.05-1 wt. %) the standard polyalkylmethacrylate dispersants such as the tetrapolymer of dialkylaminoalkylmethacrylate, C.sub.1 -C.sub.6 methacrylate, C.sub.10 -C.sub.15 alkyl methacrylate and C.sub.16 -C.sub.20 alkyl methacrylate while generally providing excellent dispersant activity undesirably degraded rust protection of the low ash crankcase oils. This tendency to degrade is masked in the high ash content oils in that the calcium rust inhibitor compounds therein compensate for the deteriorating effect of the polymethacrylates, this excess not being present in the low ash oils. Accordingly, there was need in respect to the low ash oils to develop an ashless dispersant-VI improver which did not degrade the rust protection of the crankcase oil formulation of which it is a part.
The terms "fully formulated" and "finished" employed hereinbefore and hereinafter denotes a crankcase oil which contains as a minimum, additive(s) which supply(s) anti-rust, anti-corrosion (non ferrous), and dispersancy properties to the oil formulation. The fully formulated compositions usually contain, but not necessarily, additional additives such as supplementary dispersant, anti-oxidant, anti-wear agent and anti-foamants. It is to be noted a single additive may have multiple properties such as anti-rust, corrosion inhibition and dispersancy and thus a crankcase oil containing only such an additive would be deemed "fully formulated" or "finished." Further, when referring to ash, it is intended to denote the amount of inorganic ash in oil formulations left after combustion and treatment with sulfuric acid. Still further, the term "anti-rust" refers to ferrous metal protection and the terms "corrosion inhibition" or "corrosion inhibitor" refers to protection of non ferrous metals.